1. Field of the Invention
The present invention relates generally to digital modulators, and more particularly, to a digital modulator used as a MODEM for a digital communication equipment such as a land mobile radio telephone, a portable radio telephone and a cordless telephone.
2. Description of the Background Art
A conventional digital communication apparatus modulates a carrier signal in response to a digital information signal (baseband signal) to transmit the information signal in order to achieve efficient transmission.
Such modulation systems include an amplitude modulation system wherein an amplitude of a carrier signal is changed in response to a digital baseband signal (a modulating wave signal), a frequency modulating system wherein a frequency of a carrier is deviated in response to a modulating wave signal, a phase modulating system wherein a phase of a carrier is changed in response to a modulating wave signal and an amplitude phase modulating system wherein an amplitude and a phase of a carrier are individually changed in response to a modulating wave signal.
The carrier signal (modulated signal) S(t) thus modulated in response to a modulating wave signal can be generally expressed by the following equation. ##EQU1##
Herein, A(t) denotes an amplitude, .omega..sub.c denotes a carrier frequency and .phi.(t) denotes a phase of a modulating wave signal.
As is clear from the above-described equation (1), the modulated signal can be represented by two components orthogonal to each other, that is, by a sum of an in-phase (I phase) component (the first term of the above-described equation (1)) and a quadrature phase (Q phase) component (the second term of the above-described equation (1)). Such a modulated signal can be therefore formed by using a quadrature modulator.
FIGS. 1 and 2 are a block diagram and a spatial diagram schematically showing the principle of such a quadrature modulator, respectively. It should be noted that the following example shows a phase modulating system for changing a phase of a carrier in response to a baseband signal, wherein an amplitude A (t) is fixed to "1".
With reference to FIG. 1, a mapping circuit 2 outputs I phase and Q phase components of a modulating wave signal as rectangular signals i(t) and q(t) in response to a digital baseband signal applied through an input terminal 1. The I phase component i(t) is applied to one input of a multiplier 7 through a low pass filter (LPF) 3, while the Q phase component q(t) is applied to one input of a multiplier 8 through a low pass filter LPF 4.
A carrier signal cosset is applied from a signal source 5 to the other input of the multiplier 7 which outputs an I phase component cos.phi.(t).multidot.cos .omega..sub.c t of a modulated signal. A signal -sin.omega..sub.c t obtained by shifting the phase of the carrier signal from the signal source 5 by .pi./2 by means of a phase shift circuit 6 is applied to the other input of the multiplier 8 which outputs Q phase component -sin.phi.(t).multidot.sin.omega..sub.c t of the modulated signal. Thus obtained I phase component and Q phase component can be represented in a one-to-one correspondence on the I and Q coordinates as shown in FIG. 2.
These I phase component and Q phase component are added to each other by an adder 9 to become such a modulated signal as expressed by equation (1), which signal is output from an output terminal 10.
The above described mapping circuit 2 includes two ROMs wherein signal waveform data, which have been obtained in advance by calculation, of I and Q phases of the digital modulating wave signal with their bands being limited are stored, respectively. Such waveform data are read out from the ROMs using the digital baseband signal applied through the input terminal 1 as addresses. Digital data read out from the ROMs for respective ones of the I phase and the Q phase are converted into analog signals separately by means of D/A converters contained in the mapping circuit 2 to be supplied as the above described signals i(t) and q(t).
There is a case where M-phase PSK (Phase Shift Keying) signal is generated by using such a quadrature modulator. FIG. 3 is a diagram schematically showing the principle of the generation of .pi./4 shift QPSK (Quadrature Phase Shift Keying) signal, which signal is one example of such a M-phase PSK signal.
With reference to FIG. 3, it is assumed that a signal point corresponding to I phase component and Q phase component data of a baseband signal (modulating wave signal) at a certain time point exists at one of points a, c, e and g on the unit circle having a radius of 1 shown in FIG. 3. At a subsequent time point after a lapse of a predetermined time slot, the signal point shifts to one of the intersections b, d, f and h between two virtual axis obtained by rotating the I axis and the Q axis by .pi./4 and the unit circle of a radius of 1. The I axis and the Q axis will be rotated by .pi./4 for each predetermined time slot in the same manner as described above, whereby the signal point sequentially shifts on the unit circle.
For example, if the signal point initially exists at the point a in FIG. 3 and the baseband signal does not change, the signal point shifts as a point.fwdarw.b point.fwdarw.c point.fwdarw.d point.fwdarw.e point.fwdarw.f point.fwdarw.g point.fwdarw.h point for every predetermined time slot, that is, at every .pi./4 rotation of the I axis and the Q axis. In this case, the I and Q phase data each takes the five types of values such as "1", "1/.sqroot.2", "0", "-1/.sqroot.2+ and "-1" as can be seen from FIG. 3.
On the other hand, according to the digital cellular telecommunication system standard of Japan (RCR) and the cellular telecommunication standard (TIA-IS-54) of the North America, differential encodings are carried out in .pi./4 shift QPSK modulation. Because of such differential encoding, it is only necessary to consider a relative phase between continuous symbols. Therefore, by shifting the phase of the signal spatial diagram of FIG. 3 by .pi./8 as shown in FIG. 4, data of the I phase and the Q phase have levels of four values. Such .pi./4 shift QPSK modulation by such differential encoding is generally referred to as ".pi./4 shift DQPSK modulation".
On the other hand, a digital quadrature modulator has been proposed wherein multiplication data of a baseband signal i(t) of the I phase and one carrier signal cos.omega..sub.c t has been calculated in advance and stored in one ROM for the I phase and multiplication data of a baseband signal q(t) of the Q phase and another carrier signal -sin.omega..sub.c t has been calculated in advance and stored in another ROM for the Q phase and outputs of the respective ROMs are added to each other and a result of addition is converted into an analog signal. Such digital quadrature modulator is disclosed in U.S. application Ser. No. 823,246 filed Jan. 21, 1992, now U.S. Pat. No. 4,225,795 and commonly assigned with the present invention. According to such disclosed technique, only one D/A converter is sufficient as compared with the conventional example shown in FIG. 1. In addition, since the quadrature modulation is carried out in a digital manner, there is another advantage that no vector errors are caused as compared with a case wherein the quadrature modulation is carried out in an analog manner as shown in FIG. 1.
According to such digital quadrature modulation technique, however, even if each of the multiplication results i(t) cos .omega..sub.c t and -q(t) sin .omega..sub.c t of the baseband signals and the carrier signals is "0", such data "0" have to be stored in ROMs for the I phase and the Q phase, resulting in difficulty in reducing ROM capacity. Such difficulty in reducing the ROM capacity also makes it difficult to implement the modulator itself as an LSI circuit and also increases its manufacturing cost.
In a conventional digital modulator, on the other hand, it was not considered how to cope with burst transmission. Burst transmission without considering any countermeasure results in generation of spurious (undesired) transmissions.
More specifically, in the normal burst transmission, the data transmission in effected intermittently as shown in FIG. 5A(a). As shown therein, if the time width of transmission is defined as T.sub.B (sec), the spectrum as expressed by the following equation is generated. ##EQU2##
FIG. 5B is a graph showing such spectrum wherein a spurious transmission is caused by the portion indicated with hatching.
In order to prevent generation of such spurious transmissions, a generally-called ramp processing for smooth rise and fall of burst as shown in FIG. 5A(b) is required.
FIG. 5A(c) is an enlarged diagram showing transmission waveform in such rising and falling. The following function is used as a function for rising. ##EQU3##
The following function is used as a function for falling. ##EQU4##
In the above equations (3) and (4), "T.sub.s " indicates the symbol period.
However, a conventional system needs an additional ROM dedicated for such a ramp processing. A digital modulator applicable to burst transmission by a conventional system inevitably requires an ROM of a larger capacity.